bleeding brakes

I have had to tap the brake pedal a couple of times to get a proportion valve to cooperate with gravity bleeding. Most of the time I just wait it out.
If you understand the function of a P trap under a sink, you can picture what will happen in any brake line bend that resembles an inverted P trap. A tiny air pocket can remain there. If that spot should be as high or higher than the fluid level in the bowl, the bubble is more likely. This is why gravity bleeding on some vintage Corvettes simply will not work.